There are numerous situations, on both the military and civilian sectors, where assorted equipment includes high impedance electronic circuitry. Such circuits, either in their entirety or modular segments thereof often take the form of a dielectric fiberglass-epoxy resin circuit board provided with numerous apertures, often very close to each other, to receive interconnected circuit elements that operate at relatively high potential differences with respect to each other. Because of the high impedance of the circuit as a whole, there is usually only a very small heat generation during operation of the elements, and thus not enough thermal energy to evaporate any water that may condense on the board surface. Thus any condensation on the circuit board surface or on component portions adjacent thereto could cause electrical shorting between circuit elements. Furthermore, the touching by human hands of any portions of the board surface may leave thin deposits of body salts of organic material that, with ambient moisture, can provide electrical paths between adjacent electrical elements which should not be subjected to extraneous stray currents between each other. Any such residue or contamination, therefore, could be as harmful as actual condensation on the circuit board.
Therefore, it is common practice to apply conformal coatings to portions of such circuits, especially to the circuit board to which the elements are attached, to inhibit chemical corrosion, fungus growth, physical breakage from mechanical fatigue or shock, and more often to limit electrical conduction between conductive parts of the circuit under high relative humidity ambient conditions. Any organic salt residues or other ionizable contamination that locates on the circuit board becomes trapped under the conformal coating and is thereby sealed away from environment moisture and, therefore, will not ionize to provide a conductive path. Under certain circumstances, copper dendrites may tend to grow from and between individual components or the sockets into which they are plugged into the circuit board, and such spurious outgrowths can create serious problems. To avoid such problems, therefore, it is very important to be able to apply a thin conformal coating to elements that are to be protected from humidity and possibly chemical corrosion under circumstances involving vibration, temperature changes and shock.
Most known conformal coatings are viscous compositions that are readily applied to the desired surface with a trowel like device, e.g., a human finger, a wooden spatula or the tip of a small screw driver. The major problem in applying such a coating in this manner is that the applicant cannot ensure that the thickness of the coating does not fall below a certain critical minimum value.
Many commercially available conformal coatings for such purposes contain silicon compounds or resins that harden after application and are hard to remove. Therefore, when such a circuit board has to be worked on, e.g., by the removal or replacement of one or more elements mounted thereon, it would be advantageous if a known solvent applied to a conformal coating can be used to dissolve it away from the surface to which it adheres. Such a solvent may conveniently be contained in a pressurized can provided with a pressure actuated aerosol-type nozzle to which is attached a short small-bore tube to direct the solvent to the precise part at which the coating is to be dissolved. Well known examples of such packaging are automobile carburetor cleaning solvents. The ejection of a propellant gas with such a solvent assists in blowing away the dissolved coating and leaving a clean surface.
A need, therefore, clearly exists for a composition that is at least as convenient to use as known coating compositions, is easily removable preferably with commonly available solvents, and yet can readily be applied to have a minimum thickness not less than some predetermined value to ensure that there are no inadequately protected zones at which the surface impedance falls to such a low value that electrical problems can arise.